1. Field of the Invention
The present invention relates to oscillators using a negative resistance element, and it particularly relates to an oscillator for oscillating high frequency electromagnetic waves (hereinafter, called terahertz waves) having an arbitrary frequency band from a millimeter-wave band to a terahertz-wave band (equal to or higher than 30 GHz and equal to or lower than 30 THz).
2. Description of the Related Art
For many organic molecules of biological materials, medicines and electronic materials, for example, terahertz-wave frequency regions may have absorption peaks derived from their structures and states. Terahertz waves may have high permeability to materials such as paper, ceramics, resins, and fabrics. In recent years, imaging technologies and sensing technologies using such features of terahertz waves have been studied and developed. For example, applications to safer fluoroscopic inspection devices instead of X-ray systems and in-line non-destructive inspection apparatuses in preparation processes have been expected.
An oscillator having a resonator integrated with a negative resistance element has been well known as a current injection light source that generates electromagnetic waves in such frequency regions. Japanese Patent Laid-Open No. 2007-124250 discloses a terahertz wave oscillator including a negative resistance element having a double-barrier resonant tunneling diode (RTD) and a resonator having a slot antenna, which are integrated on a conductive substrate. The oscillator uses a differential negative resistance based on intersubband transition of electrons within a semiconductor quantum well structure of the RTD to oscillate terahertz waves at room temperature.
Japanese Patent Laid-Open No. 2006-101495 discloses a terahertz-wave oscillator including an RTD and a microstrip resonator integrated on one substrate.
As disclosed in Japanese Patent Laid-Open No. 2007-124250, such an oscillator using a negative resistance element may require a bias circuit to adjust bias voltage of the negative resistance element. A structure may further be required to cause a bias circuit to have lower impedance in a frequency region equal to or higher than DC and lower than fOSC so that low frequency parasitic oscillations due to the bias circuit may be suppressed. Here, the term fOSC refers to a terahertz wave frequency (oscillation frequency) oscillated by an oscillator, and the term “parasitic oscillation” refers to parasitic oscillation in a lower frequency band that is different from a desirable oscillation frequency fOSC of an oscillator.
On the other hand, connection of such a bias circuit and a structure for suppressing parasitic oscillation may result in leakage and losses of terahertz waves at the oscillation frequency fOSC within the resonator, which may reduce the oscillation output. For example, Japanese Patent Laid-Open No. 2007-124250 proposes a method for providing a stub line across a slot antenna for reducing such losses. An oscillator using a microstrip resonator as disclosed in Japanese Patent Laid-Open No. 2006-101495 may also require a structure for reducing terahertz wave losses of oscillation frequency fOSC.